This invention relates to power supplies for guitar amplifiers and, more specifically, to power supplies for vacuum tube guitar amplifiers wherein a plurality of rectifier devices are automatically selected to operate in conjunction with varying power supply loads, as determined by the number of selectable output tubes in operation, to provide enhanced dynamic response under all performance conditions.
Electric guitar playing can be a tactile experience in addition to being an auditory experience. A properly configured amplifier and power supply can greatly enhance the dynamic playing experience by emphasizing impact and expressive nuance and “feeding it back” through the musician's ears such that the tactile sensation of playing is greatly enhanced. At their best, these enhance amplifier dynamics can provide the kind of inspiration musicians might call “black magic” because it enables them to actually play better. One important contributor to expressive dynamic response in a vacuum tube guitar amplifier is the voltage drop under variable musical demands of the high voltage power supply. The very notion of voltage drop in the power supply is counter to the prevailing concept which has always held that a stiff, well regulated power supply is essential for reducing all types of distortion and providing accurate amplifier response. However, electric guitar amplifiers may prove the exception to this rule because, as true musical instruments, their subjective characteristics often matter more than the purely objective and measurable ones for reasons as set forth hereinbelow.
The actual value of the high voltage itself exerts a notable influence on the “voice” of an amplifier, this is, the spread of harmonic emphasis that gives each note its timbre and contributes to the sonic identity of each amplifier design. Going a step further, an amplifier can be made to exhibit a mysterious twisting, morphing quality to the harmonic emphasis of even a single note, partly as a result of the changes occurring in power supply voltages. Before a note is plucked, the high-voltage filter capacitors in the power supply may be assumed to attain a state of full charge. At the instant the guitar string is plucked, the attack of the note will be emphasized as the filter capacitors unload their peak storage energy. Shortly thereafter, controlled voltage drop followed by voltage rebound can be made to occur. Taken as a whole, these power supply dynamics contribute notably to the complex twisting and morphing characteristic which can give a fine musical amplifier its elusive complexity and inspirational sound plus the enhanced tactile feel.
This has been a subject of great interest and has been the subject of prior U.S. Pat. No. 4,713,624 which teaches an amplifier with improved dynamic characteristics and U.S. Pat. No. 5,168,438 which describes selectable rectifier devices in an amplifier power supply and is more closely related to the subject invention. In should be understood that not all musical situations benefit from elastic, dynamic power supplies and to that end the alternative of switchable silicon diodes may at times be preferable to vacuum tube rectification, as specified in the above-noted '438 patent. The voltage drop across a silicon rectifier is a mere half volt and is unchanging under dynamic loads. In contrast, the vacuum tube rectifier may exhibit substantial voltage drop which can increase rather dramatically under the fluctuating load demands of the output amplifier. However, too much voltage drop would indicate a rectifier tube operating outside its safe parameters, whereas to little sag or drop would reduce the potential for enhancing dynamic response in a guitar amplifier. Therefore, the correct amount of voltage drop in the power supply can only be achieved by carefully matching the characteristics of the vacuum tube rectifier with the dynamic demands of the amplifier it serves. It must also be understood that voltage drop under dynamic conditions first requires a power amplifier which places varying current demands on the power supply. Thus, a Class A amplifier, with its steady current draw, is a very poor choice for this type of power supply refinement. Other less important elements may also contribute to the high voltage sag or drop under dynamic power conditions, notably the power transformer. Once again, though a transformer which, by itself, produced musically desirable dynamic characteristics, would probably run too hot for reliability.
Furthermore, the problems of properly matching power supply dynamics are exacerbated in amplifiers which offer the user a selection in the number and/or type of output tubes to be utilized. In the amplifier described in U.S. Pat. No. 5,559,479, different types of output power tubes may be operated individually or in various combinations to alter both the sonic character and the output capability. Thus, the switchable power amplifier characteristics of such an amplifier create the need to properly match power supply characteristics under all conditions.